It is often desirable to determine the location and/or movement of a person or other object within a home, business, hospital, prison, or other area or space. For example, elderly or infirm persons living alone or in a nursing home may need frequent or even relatively constant monitoring by caregivers. As another example, firefighters and other first-responders in a risky environment, such as a burning building, may need to be tracked to increase or ensure their safety.
Various techniques have been used to determine the location and movement of persons or other objects, such as radar, Global Positioning System (GPS), Time-of-Flight, and Time-Difference-of-Arrival techniques. For example, radar techniques involve the reflection of electromagnetic radiation off a person or other object and may be used for tracking purposes. GPS techniques, where the person or other object is equipped with a GPS transmitter for transmitting signals that can be received by GPS satellites, may also be used for tracking purposes. However, these techniques require relatively expensive equipment and may not work effectively under certain circumstances. For instance, radar and GPS systems may not work effectively within a building because the walls and ceilings of the building can block radar signals or block access to GPS satellites.
Relatively less expensive radio techniques may also be used to determine the location and movement of a person or other object. For example, the power level of a radio signal typically attenuates at a logarithmic rate over distance. A radio receiver can receive a signal transmitted by a radio transmitter, measure the received signal's power level, and estimate a distance between the radio transmitter and the radio receiver. Using signals from several radio transmitters at different known locations, a mobile radio receiver could determine its location using triometric or multilateration calculations. However, approaches based on signal power attenuation often produce accurate location estimates only in a perfect world devoid of radio interference or physical objects, which can absorb or reflect radio signals. Also, since the power level of a radio signal attenuates at a logarithmic rate, the accuracy of the distance calculations in these techniques typically deteriorates exponentially as the signal power level decreases. As a result, the accuracy of the estimated location using these approaches can be significantly affected.